sub FirstIfKeys {
    my $hv= pop @_;
    my %k;
    @k{ keys %$hv }= (1) x @k;
    my @k;
    for my $key (  @_  ) {
        push @k, $key   if  delete $k{$key};
    }
    return @k, keys %k;
}

    for my $key (  FirstIfKeys( "Foo", \%HoA )  ) {
        ...
    }